Structure and grain growth of TiO2 nanoparticles investigated by electron and X-ray diffractions and Ta-181 perturbed angular correlations

Abstract

Bare and coated TiO₂ nanoparticles with particle sizes d < 5 nm have been synthesized in a microwave plasma process. Structural properties of these materials have been investigated by transmission electron microscopy, x-ray diffraction, and perturbed angular correlation (PAC) measurements of the electric quadrupole interaction (QI) at the probe nucleus ^181Ta on the metal site of TiO₂ at temperatures 290 ≤ T ≤ 1450 K. The electron diffraction of the uncoated nanoparticles in the as-synthesized state reflects long range order in the Ti sublattice. Depending on the particles size, either the anatase or the rutile phase of TiO₂ was found. Anatase appears to be the stable form of nanocrystalline TiO₂ below d ~ 10 nm. The PAC spectra of these nanocrystalline oxides are characterized by a broad distribution of strong quadrupole interactions, indicating a strongly disordered oxygen environment of the metal sites. Upon annealing, the grain size grows from d < 5 nm after synthesis to d > 100 nm after 1300 K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO₂. The data suggest a critical grain size of d ~10 nm for the onset of the ordering process. The spectra of coarse-grained TiO₂ are reached at a particle size d >= 30 nm. In n-TiO₂ coated with Al₂O₃ and ZrO₂ both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO₂ cores and in the ZrO₂ coating of n-TiO₂/ZrO₂ at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO₂ kernels and the coatings.